Recently, technical advances in OA equipment such as copying machines and page printers are progressing with relation to color, precision, and digitalization. In accordance with such advances, motors used in such equipment must be able to operate with high rotational speed accuracy over a wide range of rotational speeds. Further, control circuits for controlling such motors must be equipped with optimal control functions at every number of rotations over a wide range of rotational speeds.
Conventionally, a digital servo circuit 100 as shown in FIG. 3 has been proposed for digitally controlling the speed and phase of a motor (for example, refer to Patent Literature 1). The digital servo circuit 100 shown in FIG. 3 is an example of a motor control circuit that controls the speed and phase of a drum motor 102 and a capstan motor 103 used in VTR.
To a servo IC 117 of the digital servo circuit 100, detection signals corresponding to the speed and phase of the drum motor 102 are respectively supplied from terminals 104a and 104b, an output from a speed servo circuit 119 is supplied to a mixer 127 via a multiplier 123, and an output of a phase servo circuit 120 is supplied to the mixer 127 via a digital filter and a multiplier 24. A PWM signal that appears in an output of the mixer 127 is added to the drum motor 102 via a low-pass filter 109 and a drive amp 115 (the same applies to the capstan motor 103, and thus an explanation thereof will be omitted).
The digital servo circuit 100 is configured such that control gains (i.e. coefficients KDS, KDP, KOS, and KOP) corresponding to the VTR operation states of speed servo circuits 119 and 121 and phase servo circuits 120 and 122 are furnished from an external microprocessor 118. Thereby, the adjustment location on the output side of the servo IC 117 is eliminated and the number of parts is reduced, and thus high performance and automatic adjustment are achieved in the digital servo circuit 100.
In addition, in a motor control circuit, it is known that when performing control of speed that changes over a wide range, it is necessary to set an optimal control gain for each speed within the speed variable range. Referring to FIG. 4, a typical example of such control gain setting will be explained below.
In a motor control circuit 200 shown in FIG. 4, a speed error signal SD corresponding to a deviation between a detected rotational speed of the motor and a target speed and a phase error signal PD corresponding to a deviation between a detected motor phase and a reference phase are added via a speed input resistor RPD and a phase input resistor RSD, and then the signals after addition are integrated by an integration amp 201. Thereby, a control signal (for example, a torque command signal) for a subsequent motor drive circuit (not illustrated) is obtained.
The motor control circuit 200 includes a low speed-side integration constant circuit (a resistor RZ1 and a capacitor CZ1) that determines a control gain when the motor is rotating at a low speed, and a high speed-side integration constant circuit (a resistor RZ2 and a capacitor CZ2) that determines a control gain when the motor is rotating at a high speed. One end of the low speed-side integration constant circuit is connected to a low speed integration output terminal INTO1, and one end of the high speed-side integration constant circuit is connected to a high-speed integration output terminal INTO2. The other end of the low speed-side integration constant circuit and the other end of the high speed-side integration constant circuit are connected to a common integration input terminal INTI. The motor control circuit 200 also includes a selection switch 202 that switches which of the low speed integration output terminal INTO1 and the high-speed integration output terminal INTO2 is connected to the integration amp 201.
Here, in the motor control circuit 200, the control gain is normally selected by a signal input from the outside through an interface. The selection switch 202 switches between the low speed integration output terminal INTO1 and the high-speed integration output terminal INTO2 in response to a switch signal SW that is associated with the signal input from the outside. Thereby, appropriate control gains are set during low speed rotation and high-speed rotation of the motor.